Silicon is the second-most element of the earth's crust (more than 80% of the earth's crust consist out of silicates) and is present in all forms of different compounds. Silicon compounds do not only represent most of the species of this class of minerals, but are also very important from an economical point of view. They are used in large scales and diverse forms. Glass, porcelain, emaillie, clay products, cement and water glass are technically important materials that consist out of silicates. The catalytic properties of some of the silicates are used synthetically. Their versatile uses are further expanded, if other elements, in particular aluminium, occupy some of the lattice positions that are otherwise occupied by silicon. Feldspars and zeolithes, for example, belong to these alumo silicates; the importance of the latter is based in particular in their molecular sieve and ions exchange properties. Al- and Ca-silicates have become important as filling materials in the laque-, rubber-, plastics- and paper-industry, Mg-silicate (talcum) as an absorber and filling material in cosmetics and pharmaceuticals and alkali-aluminium-silicates as exchange for phosphates in cleaning agents. For the setting of Portland cement, silicates play an important role. Since some silicates carry free OH-groups (analogous to the silanoles) on their surfaces, one can bind reactive groups thereto; these properties are used for immobilisation in the solid phase-technique.
1. 1. Silicon Dioxide
Silicon dioxide (SiO2) is a solid with a high melting point, which is present both in crystallised and amorphous forms. In all these forms of appearances, each silicon atom is tetraedrically surrounded by four oxygen-atoms (coordination number: 4). When crystallised, silicondioxide is present in different modifications (quartz, tridymite, cristobalite and others). The most common form of crystalline SiO2 is quartz. Amorphous silicon dioxide minerals are, amongst others, achat, opal and flintstone. Quartz glass that is obtained by melting of quartz and slow cooling of the melted material does not anymore exhibit crystal surfaces. It is used for, amongst others, the production of quartz lamps (because of its permeability for ultraviolet radiation), and heat resistant apparatuses. Furthermore, the shells of diatoms (diatomeen) consist out of amorphous SiO2. 
1.2. Silicic Acids and Silicates
Silicon exhibits the coordination number 4 also in silicic acid and silicates. The tetraedrically-built [SiO2]4-ion tends to polymerisation by of SiO4-subunits. In this case, two Si-atoms are bound to another linking by one O-atom.
From ortho-silicic acid, at first ortho-disilicic acid (pyro-silicic acid; H6Si2O7) is formed by splitting off water (condensation). Further condensation leads to the meta-silicic acids [(H2SiO3)]n via the poly-silicic acids. In case of smaller numbers of SiO4-units (n=3, 4 or 6), by this also ring-shaped molecules can be formed.
The poly-silicic acids have an amorphous (unordered) structure.
The salts of the ortho-disilicic acids (ortho-silicates), having the structure Me2SiO4, contain single [SiO4]4− anions. The water-soluble alcalisilicates which can be obtained, for example, by melting of quarz with soda, brine or potassium carbonate, in addition to [SiO4]4− anions, contain also [Si2O7]6− and [Si3O10]8− anions (and larger anions). Such “water glass” solutions, from which the solubilized particles can be separated by dialysis at a membrane are suited, amongst others, for the cementation of glass and porcelain, for the impregnation of paper, as flame protective agent for wood and for the conservation of foods.
After the acidification of such an alkalisilicate solution, the acid molecules that have been formed out of the [SiO4]4− and [Si2O7]6− groups (and larger groups) by proton uptake, condensate with each other to form poly-silicic acids (see above), whereby the solution becomes gel-like. The polymers obtained at first consist out of chains or networks. Upon further progress of the condensation, three-dimensional structures are formed, which correspond to the composition SiO2.
The following classification can be obtained:
1. Silicates with discrete anions:                a) Island-silicates: These are ortho-silicates with the anion [SiO4]2−. Example: phenacit, olivin, zirconium.        b) Group-silicates: The SiO4-tetraeders are linked to form short chain units. Examples: di-silicates with the anion [Si2O7]6− and tri-silicates.        c) Ring-silicates: The SiO4-tetraeders are arranged in ring form. Examples: benitoid (3-ring), axinite (4-ring), beryll (6-ring).        
2. Chain-silicates and ribbon-silicates. Chain-silicates consist out of chain-like SiO4-tetraeders bound to each other; they are polymers of the anions [SiO3]2−. By linking several SiO4-chains, ribbon-like molecules can be formed. Examples: hornblende, asbestos.
3. Layer-silicate (sheet-silicate): Layer-silicates contain even sheets made of SiO4− tetraeders. These are held together by cations stored in-between. They are polymers of the anions [Si4O10]4−. Examples: talcum, caolinit.
4. Scaffold-silicates: In the scaffold-silicates, the tetraedic SiO4-groups are bound to three-dimensional lattices. Examples: different modifications of silicon dioxide, like feldspatuses.
The condensation process that leads to the polysilicic acids or polysilicates, respectively, can be controlled by partial replacement of the OH-groups of the silicic acid by single-binding organyl residues, which do not participate in the condensation process (production of different silicones).
Synthetic silicic acids are amorphous, non-poisonous, and, in contrast to the crystalline SiO2 modification, do not lead to the generation of a silicose.
General literature:
Hinz, Silicat-Lexikon (2nd vol.), Berlin: Adademie Verl. 1985
Liebau, Structural Chemistry of Silicates, Berlin: Springer 1985
Petzold and Hinz, Einführung in die Grundlagen der Silicatchemie, Stuttgart: Enke 1979
CD Römpp Chemie Lexikon—Version 1.0, Stuttgart/N.Y.: Georg Thieme Ver lag 1995
1.3. Siloxanes, Silicones
The methods that are used for the demonstration of silanoles and siloxanes according to the state of the art consist in that water is acting on organic silicon derivatives, such as trimethyl silicium chloride [(CH3)3SiCl], whereby first silanoles, such as trimethyl silanole, are formed:(CH3)3SiCl+H2O→(CH3)3Si—OH+HCl
From these, siloxanes, such as hexamethyl disiloxane [(CH3)3Si—O—Si(CH3)3] are generated by splitting off water:2(CH3)3SiOH→(CH3)3Si—O—SiCH3)3+H2O
Furthermore, compounds of high molecular weight with ring- or chain-like structures or three-dimensionally cross-linked macromolecules (“silicones”) can be produced by reacting dimethyl or monomethyl silicon chloride with water (via the intermediate products dimethyl silanediol or methyl silanetriol, respectively):(CH3)2SiCl2+2H2O→(CH3)2Si(OH)2+2HCln(CH3)2Si(OH)2→(CH3)2SiOn+nH2O
The starting compounds R3SiOH (silanoles), R2Si(OH)2 (silandioles) and RSi(OH)3 (silantrioles) as used for the demonstration of additional silicones, are commonly produced by hydrolysis of the respective halogene compounds R3SiCl2, and R2SiCl3 (R=ethyl, propyl, phenylgroups and others).
Accordingly, siloxanes (silicones) can be grouped into:    a) linear polysiloxanes of the type R3SiO[R2SiO]nSiR3.    b) branched polysiloxanes which contain tri-functional or tetra-functional siloxane-units at their branching sites.    c) cyclic polysiloxanes which are built of bi-functional siloxane-units.    e) crosslinked polymers, wherein chain- or ring-form molecules are linked into two- or three-dimensional networks.
The viscosity of the high molecular weight silicones (silicone oils), which consist of chain-form macromolecules, increases with increasing chain length. Silicones play an important role as technical materials. Chains that are cross-linked to a low extent exhibit rubber-elasticity (silicone rubber; use: sealings and others), silicones that are highly cross-linked are resin-like (silicone resins).
Due to their hydrophobic (water-repellent) properties based on their organic portion, silicones are used for impregnation purposes (of textiles, paper and others).